Environmental concerns have prompted government regulations to curb emissions from internal combustion engines in motor vehicles. Maximum levels of various gases, such as hydrocarbons, that may be emitted from the exhaust system of the motor vehicle are strictly regulated. As such, many attempts have been made to control exhaust system emissions.
One such attempt includes the use of a catalytic converter. In a motor vehicle, a catalytic converter is used to burn off excess emissions from the engine before the exhaust gases exit through the tailpipe. Unfortunately, immediately following a cold engine start, the catalyst of the catalytic converter can be ineffective since the catalyst requires a period of time to warm up to a temperature at which the catalyst can operate effectively to burn excess hydrocarbons. As a consequence, after engine start up, hydrocarbon emissions may initially be high due to a low temperature catalyst. To add to the problem, excess fuel in the catalyst at start up may further cool the catalyst, thereby requiring an extended period of time for the catalyst to warm up to a sufficient operating temperature.
Another attempt includes the use of fuel injectors. In a motor vehicle, fuel injection and engine control strategies are aimed at minimizing exhaust emissions while maintaining engine performance and economy. Conventional fuel injectors are typically controlled by a fuel injection pulsewidth signal in which the pulsewidth determines the amount of fuel injected into the corresponding cylinder of the engine. The fuel injection pulsewidth signal is tailored to follow a programmed target fuel injection curve. The curve is programmed to minimize emissions from the engine during vehicle operation. For example, a stoichiometric air/fuel ratio is used during most operations to reduce hydrocarbon emissions. Further, spark ignition timing can be varied in order to minimize emissions. While these methods may work well during engine operation, they do not address the high emissions that sometimes result after engine shutdown and subsequent restart. (Such as the catalyst cooling described above).
Conventional engine shutdown involves synchronized deactivation of fuel delivery and ignition events. In actuality, these deactivations often do not occur simultaneously; for example, fuel may be delivered to one or more of the cylinders after the final ignition event for that cylinder. This unburned fuel may then pass through the engine and enter into the exhaust system including the catalytic converter. After engine start up, the excess fuel slows the warming of the catalyst and high hydrocarbon emissions may result.
It is therefore desirable to provide a method of minimizing the amount of fuel delivered to the exhaust system after engine shutdown in order to reduce hydrocarbon emissions.